In cases where soil is extremely dry or wet, the Vinduino measurement becomes less accurate. This is due to the Arduino 10-bit ADC (voltage measurement) resolution, that will become an increasingly stronger factor in the extreme corners of the measurement range.

The graph on the left shows how the measurement accuracy changes with the sensor resistance value. A lower reference resistor value gives better accuracy in the lower sensor resistor range, and for better measurement accuracy at high sensor resistance a higher value reference resistor helps.

The goal of Vinduino is to help with irrigation management, so we are not really interested in high accuracy when the soil is totally soaked or bone dry, beyond the point of permanent plant wilting. We need best accuracy at the target soil moisture level. With this data in mind, my choice is for using 4700 Ohm, as this gives good accuracy in the most common soil moisture range. With the described sensor design, 100% moisture gives a sensor value around 300 Ohm, at 50% soil moisture the sensor reads 1 kilo Ohm. Refer to the Vinduino sensor calibration blog post for more details.Whatever resistance value you choose for the R1 and R2 resistors connected to the sensor, should be declared in the software code. So for using 4k7 Ohm resistors (4700 Ohm), the Arduino code should be changed to:const long knownResistor = 4700; // Constant value of known resistor in OhmsFor ultimate accuracy, as resistors usually come with a tolerance of 5% or 1%, you can measure the real resistance with a multimeter and enter that as known resistor value.

Checking the accuracy based on ADC resolution for a reference resistor of 4700 Ohm, shows better than 3% measurement accuracy for soil moisture between 80%-20%, and 1% accuracy for the lowest moisture level (where irrigation should switch on). This should be good enough for our purpose.

Just some pictures this time showing the family and friends at work with the pump and the basket press. This year we let the wine sit in the fermentation bin a little longer. Will see if this is an improvement. Anyways. Wine is in the tanks, grapes are sold. We can relax a bit now and enjoy our private reserve from previous vintages!